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Friday, August 14, 2015

Finally, Some Real Rain.

For almost a week the models have been advertising a significant rain event today (Friday). And the most recent forecast is no different. A substantial rain event that will substantially reduce water use for several days and perhaps put some meaningful water into reservoirs. The first significant rain of the summer.

Let me show you the latest UW WRF model forecasts, starting with the 24h totals ending 5 AM Saturday. Looking at the regional totals, we see that western WA and southern BC get doused, as does NW Oregon. Dry over the Columbia Basin and eastern Oregon.

A closer in view suggests that the north Cascades and southern BC will be hit hard, with some locations getting 1-2 inches. Puget Sound will be moistened by perhaps 1/3 inch. Your plants will be happier.

Now the above is a single, deterministic forecast. Let's get an idea of the confidence in our Friday forecast by looking at an ensemble prediction using the NWS SREF (Short Range Ensemble Forecast System) in which roughly two-dozen forecasts are made. The graphic below shows you the accumulated precipitation at Seattle Tacoma Airport for all of these forecasts. Time is on the bottom axis (in GMT). The y-axis is accumulated rain. Each color line is one model. The black line is the mean of all the forecasts, also known as the ensemble mean. On average, the ensemble mean is the best forecast.

You will notice that the ensemble mean indicates about .3 inches in total over Seattle, but there is considerable spread, and thus uncertainty, in the forecasts, with totals ranging from .05 to 1.2 inches.

25 comments:

What is the scale on the UW model map? cin = centi inches? Interesting, never seen that, but I assume common in the weather forecasting biz...

On a related note, seems like with the right weather system and track, the warm water Blob should increase precipitation in the NW due to additional moisture/convection. Or maybe the area of the Blob is too small to have much effect on passing systems.

The rain started here in Fauntleroy at 1300 and now at 1520 we are just over an inch. That top blue line may be a bit low by the end of this storm for us. With these cells moving around it looks to be a bit hit or miss as to how much rain you are going to see.

It is just a bit after 4pm, and we have received about two inches of rain in an unscientific bucket on deck measurement. We are near the University Village (which is just east of the Univ. of Washington main campus). For a while there was a river going down our street. I am so glad we put in permeable pavers on the 90 foot long shared driveway so it did not add more to the "river".

I've had about 1.25" here on Bainbridge Island since midnight, about 1.15" from the event that started after noon and 0.10" from last night's thunderstorms. Exact total will come once it stops and I can measure the overflow in my gauge.

(I have one of those COCORAHS-style gauges with an inner graduated cylinder that measures the first inch of rain in 0.01" increments. After an inch falls it overflows into the outer cylinder and you have to play empty-and-transfer to get an exact total.)

Rod, I remember a day in August 2004 when we got an inch or more of rain and there was flooding. Cliff's blog doesn't go back that far but I did some searching and according to this KOMO page it was on August 22, 2004: http://www.komonews.com/news/archive/4141256.html

Final (I think, rain has stopped, sky is lightening, and radar indicates it's basically all over for me) total at my place on Bainbridge is 1.30". Very impressive for mid-August, and very welcome. I'm about to go out and savor the woods being moist and fresh again, instead of dusty and very dry.

So yesterday was a great relief after an incredibly dry spring and summer.

Here in Fauntleroy in West Seattle we had had 1.62" of rain between April 14 and August 13. We had 1.92" yesterday, more than doubling that previous four months. Won't have to water for a week after that. A nearby weather station showed 2.1" while others not more than two miles away show as little as 1.4". We had a storm cell pass right over us and had a rain rate of around .7"/hr for a full hour. I've seen it rain that hard (or harder) here before, it has just never lasted for more than 10-15 minutes, going for a full hour was like being under a hose.